1. Field of Invention
The present invention relates generally to power amplifiers and in particular to a dual-band, dual-mode power amplifier.
2. Description of Related Art
In the United States, cellular operating licenses have been awarded by the Federal Communication Commission (FCC) pursuant to a licensing scheme which divides the country into geographic service markets. Cellular licenses were originally granted for radio frequency (RF) blocks in the 800 MHz range. Most 800 MHz cellular telephone systems in the United States utilize the Advanced Mobile Phone Service (AMPS) analog air interface standard. A later generation air interface standard for the 800 MHz band, known as D-AMPS, has subsequently been developed and implemented. The D-AMPS standard comprises both digital and analog cellular communication. Thus, there are presently both analog (AMPS) and digital (D-AMPS) cellular telephone networks in operation at 800 MHz in the United States.
In response to increased demand for cellular services, a number of digital air interface standards were developed for providing efficient digital communication of voice, data, fax and text messages under the umbrella of "personal communications services" or PCS.
The FCC recently auctioned spectrum in the 1900 MHz range for use by PCS systems. Operational PCS systems, such as systems based on the GSM TDMA (Time Division Multiple Access) or IS-95 CDMA (Code Division Multiple Access) air interface standards, are now beginning to appear in the United States. Meanwhile, existing 800 MHz cellular systems are continuing to operate.
Thus, there are presently operating in the United States analog and digital cellular systems at 800 MHz and digital PCS systems at 1900 MHz. Mobile subscribers who desire to receive services from systems operating at 800 MHz and from systems operating at 1900 MHz must either use two different mobile transceivers capable of operating within the cellular or 800 MHz band and the PCS or 1900 MHz band, respectively, or, preferably, use a single "dual-band" mobile transceiver which can receive and transmit RF signals in both frequency bands. Moreover, mobile subscribers who wish to communicate using both analog and digital systems must again either use two different mobile transceivers or, preferably, use a single "dual-mode" transceiver. Ideally, a mobile transceiver is capable of both dual-mode and dual-band operation to provide the user with maximum flexibility and functionality.
A problem arises, however, in that the power amplifier used in a mobile transceiver is typically optimized for use in a particular band (i.e. PCS or AMPS) and in a particular mode (i.e. analog or digital). This problem is manifested in two ways: as an impedance matching problem and as an amplifier biasing problem.
For maximum efficiency, the impedance at the output of the amplifier must be matched to the impedance of a duplexer/diplexer prior to transmission. However, the impedance of the matching circuit is dependent on the frequency of operation. Thus, a conventional matching circuit optimized for matching the impedance of an amplifier at 800 MHz will generally not optimally match the impedance of the same amplifier operating at 1900 MHz. Moreover, the impedance of the amplifier is dependent on the mode of operation. Thus, a conventional matching circuit optimized for matching the impedance of an amplifier operating at 800 MHz in the AMPS mode will not adequately match the impedance of the same amplifier operating at 800 MHz in the D-AMPS digital mode.
The biasing problem arises due to the fact that the efficiency of an amplifier is dependent on the mode or class of operation of the amplifier which is determined by the modulation technique employed. Typically, analog communication systems employ well known frequency modulation (FM) techniques to modulate analog information onto a carrier signal, while digital communication systems employ digital modulation schemes, such as .pi./4 DQPSK (Differential Quadrature Phase Shift Keying) modulation. Signals transmitted using frequency modulation are most efficiently amplified by a power amplifier biased and operating in non-linear or saturated mode. Signals transmitted using .pi./4 DQPSK modulation, on the other hand, are most efficiently amplified by a power amplifier biased and operating in linear mode.
One possible solution to the impedance matching problem is to provide the amplifier with separate, switched high-pass and low-pass matching networks at its output. However, the switch must be capable of handling high power, which tends to require a large, costly switch. Moreover, the match at 800 MHz will necessarily be a blended match, causing a loss of efficiency in the analog mode.
Another solution to the impedance matching problem is to provide a broadband power matching circuit covering both desired frequency bands and having peaks at the transmit bands. Such a configuration would tend to waste bandwidth, however, when the desired match frequencies differ by an octave or more and the desired bandwidth in each band is relatively narrow. The well known Fano's Limit shows that there is a physical limitation on broadband matching when a reactive element (such as the drain-source capacitance of a transistor) is present.
In transceivers having a single amplifier chain, prior art solutions to the impedance matching problem address the biasing problem by providing separate bias levels, although the impedance match remains constant. If a single amplifier is used to amplify both analog and digital signals, the amplifier must be biased to barely meet linearity requirements while retaining as much analog efficiency as possible. Such an arrangement tends to be inefficient. As radiotelephones become smaller and power consumption requirements become stricter, such inefficient operation is highly undesirable.
Therefore, there is a need in the art for a power amplifier circuit capable of efficient operation in both 800 MHz and 1900 MHz systems and in both analog and digital systems. Such a dual-band, dual mode power amplifier preferably provides an integrated, efficient solution to the problems described above.